Signal, the open‑source messaging app that has become the gold standard for private communication, has added a third ratchet to its already robust Double Ratchet. The new layer, called the Sparse Post‑Quantum Ratchet (SPQR), works in tandem with the existing scheme to create what Signal calls the Triple Ratchet. The result is a protocol that guarantees forward secrecy and post‑compromise security even if a quantum computer later breaks elliptic‑curve cryptography.
The trick is simple in principle: each time Alice sends a message to Bob, she also sends a chunk of quantum‑secure key material. Bob, in turn, replies with his own chunk. Together, the two parties continually exchange fresh secrets derived from a quantum‑safe Key‑Encapsulation Mechanism (KEM). Because the KEM uses mathematical problems that are believed to resist quantum attacks, the new secrets remain hidden even if an adversary later gains quantum‑level power.
The Triple Ratchet blends the quantum‑safe key with the familiar Double Ratchet key inside a key‑derivation function. The resulting key is as strong as the weakest link, so an attacker would have to break both the classical elliptic‑curve part and the quantum‑safe KEM part to learn the message contents. In practice, this means that every encrypted text is protected by two independent families of mathematics, one of which will stay secure long after the other is cracked.
Efficient Secrets in a Tiny Bandwidth Footprint
A key concern for any messaging service is the cost of sending extra data. SPQR solves this by packing the quantum key into a series of 64‑byte “seed” pieces that Alice and Bob can transmit simultaneously. The first 64 bytes of the encapsulation key are sent immediately, and Bob uses them to generate the bulk of his ciphertext. Alice then sends the remainder of her key, and Bob replies with the final part of his ciphertext. This incremental approach,dubbed the ML‑KEM Braid,allows the two parties to share a 32‑byte quantum secret in under a dozen message chunks, a negligible overhead compared to the 1‑kilobyte payloads that most chats carry.
Signal’s engineers also employ erasure‑coded chunking. If a network hiccup drops a few of the quantum chunks, the recipient can still reconstruct the missing pieces from the surviving ones. Only a sustained, targeted attack that removes all subsequent quantum chunks would break the protocol, and such an attack would immediately trigger a noticeable denial of service for the user. In other words, quantum‑safe security is built to be invisible and resilient against the most common forms of network interference.
Seamless Rollout Across a Global User Base
Introducing a new cryptographic primitive into a live messaging ecosystem is fraught with practical hurdles. Signal’s strategy is to let the Triple Ratchet “downgrade” on the first exchange if the other party’s client does not yet understand the new format. Alice can start a conversation with Bob using the new ratchet, but Bob’s older client simply ignores the extra quantum data. When Bob replies without the quantum header, Alice recognises that the other side is still on the Double Ratchet and switches to the older, but still secure, protocol for that session.
This approach avoids the pitfalls of forced downgrades that would otherwise cause long‑standing conversations to lag by hours or days. It also protects against a malicious intermediary that might try to force a downgrade: the quantum data is authenticated as part of the message’s overall integrity check, so removing it would break the message for anyone who could still decrypt it.
Signal plans to lock the Triple Ratchet into every new session once all clients are updated. Existing long‑lived sessions will be archived, ensuring that eventually every conversation,whether it began last month or last year,will be protected by the quantum‑safe scheme. The rollout is deliberately incremental, with each new app version automatically adding the quantum layer without changing the user interface or requiring manual configuration.
Confidence Through Formal Verification
Behind the elegant design lies a rigorous verification process. Signal’s Rust implementation is modelled in a formal analysis tool called ProVerif, which checks that the protocol satisfies forward secrecy, post‑compromise security, and quantum resilience. The models are compiled into the code every time a developer pushes a change, and any proof failure halts the build. This continuous integration of formal verification means that the codebase can evolve while remaining provably correct.
The verification also covers subtle state‑machine behaviours,such as how the protocol negotiates downgrades or upgrades,ensuring that no accidental loss of security can creep in during a future update. By embedding these checks directly into the code and proving that the assertions never fail, Signal guarantees that a crash will only occur in a genuinely invalid state, preserving both reliability and security.
When quantum computers move from laboratory curiosities to practical tools, the world will still be able to send private messages with confidence. Signal’s Triple Ratchet is a quiet, well‑tested shield that will absorb the quantum shock without disrupting the user experience. For the average person, the change will be invisible: a conversation with a friend or a colleague will still feel instant and secure, but it will also be future‑proofed against the next generation of computational power. In an era where the next headline could announce the arrival of quantum computing, the fact that millions of messages are already protected by a formally verified, quantum‑safe protocol will be a small, reassuring detail in the broader story of digital resilience.
Source: https://signal.org/blog/spqr/
